Land Subduction
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Land subduction is the process by which the ocean floor sinks beneath a deep ocean trench (underwater canyon at the edge of an oceanic plate) and back into the mantle. The pacific ocean is shrinking because it is ringed by many trenches and subductions is taking place faster and fatser than new crust is forming. the alantic ocean is expanding because it has few trench. J. Tuzo Wilson observed that the continents have cracks similar to those on the ocean floor. He propsed that the lithosphere is broken into sections called plates, which fit closely together and carry the continents, parts of the ocean floor, or both. He combined what is known about continental drift, sea-floor spreading, and plates into a single theory called plate tectonics, which states that a peices of the earths lithospehere are in constant slow motion driven by convection currents in the mantle.